This invention relates to the field of implantable medical devices, and more particularly relates to an implantable pressure sensing lead.
Pacemakers are available which are capable of varying their pacing rates in response a patient's metabolic need for cardiac output. Some such rate-responsive or rate-adaptive pacemakers utilize physiologically-based signals, such as signals from sensors for measuring the pressure inside the patient's right ventricle, intramyocardial pressure, myocardial contractility to derive a measurement of demand for cardiac output.
Sensing pressure at certain sites within the patient's body offers potential for accurate determination of a patient's metabolic needs. One of the earlier successful methods of pressure measurement was through the use of a catheter, as described in U.S. Pat. No. 3,473,386 to Neilsen et al. A disadvantage of the catheter approach, however, lies in its lack of accuracy. An implantable pressure sensor is described in U.S. Pat. No. 4,023,562 to Hynecek et al.; however, the Hynecek et al. sensor is not suitable for chronic implantation, as body fluids destroy the pressure transducer in a relatively short time.
In U.S. Pat. No. 4,936304 to Kresh et al. there is proposed a pressure sensor arrangement for controlling pacemaker function by detecting changes in the contractile state of the cardiac muscle. The Kresh et al. '304 patent suggests that a pressure sensor within the threaded distal end of a transvenous lead may be partially embedded in the myocardium, so that forces exerted on the distal end of the lead resulting from myocardial contraction may be detected. The Kresh patent contemplates introduction of a pressure sensor into the cardiac muscle at the apex of the right ventricle, in order to obtain readings correlated to intramyocardial contractility. However, the patent states that introduction of the pressure sensor into the ventricular wall is not preferable for providing a good indication of wall stress changes resulting from changing contractility, due to the thinness of the right ventricular wall.
In U.S. Pat. No. 4,407,296 to Anderson there is described a chronically implantable pressure sensor suitable for use in conjunction with a long-term implantable device such as a pacemaker. Other examples of chronically implantable pressure sensors are described in U.S. Pat. No. 4,485,813 to Anderson et al. and in U.S. Pat. No. 4,432,372 to Monroe. In the types of pressure sensing arrangements proposed in these patents, the pressure sensors must be hermetically sealed to allow chronic implantation. This tends to increase the size of the pressure sensors. Also, since the pressure sensors of the prior art are typically disposed on a lead, they may only be located in places accessible through conventional lead implantation techniques. The above-referenced Anderson '296, Anderson et al. '813, and Monroe '372 patents, for example, contemplate disposing the pressure sensor in the right ventricle of a patient, so that right-ventricular pressure readings, or right-ventricular muscle motion readings, are available.
U.S. Pat. Nos. 4,774,950, 4,899,751, 4,899,752, 4,986,270, and 5,027,816 to Cohen each contemplate locating a pressure sensor at various sites in the circulatory system, including the right atrium, the right ventricle, the left atrium, the left ventricle, or in a major artery. However, particularly in the case of the left-atrial, left-ventricular, and other arterial sites, the above-noted Cohen patents do not address the issue of how a pressure sensor may be reliably introduced into those sites in any substantial detail or address the specific construction of sensors for long term implant in such locations.
Notwithstanding the sensors described in the prior art discussed above, there continues to be room for improvement in the area of assessing a patient's metabolic need particularly for controlling pacemaker rate. In particular, there remains a need for practical pressure sensors for chronic measurement of pressure within the heart tissue, within the left ventricular cavity and elsewhere that conventional pressure sensing leads cannot readily be placed by means of the transvenous introduction of a sensor lead into the right heart.